Importance of assessing nonattenuation-corrected positron emission tomography images in treatment response evaluation of primary cutaneous lymphoma

A 62-year-old lady diagnosed with primary cutaneous anaplastic lymphoma. Baseline staging positron emission tomography/computed tomography (PET/CT) showed fludeoxyglucose (FDG) uptake in the right anterior abdominal wall on maximum intensity projection image (MIP) [Figure 1a] - thin narrow black arrows] with rest of the scan unremarkable. Fused transaxial PET/CT images [Figure 1b] and PET images [Figure 1c] show FDG uptake in the skin thickening in the right anterior abdominal wall. Response evaluation was done after two cycles of chemotherapy. Deauville's’ scoring was used to assess the for response evaluation on PET/CT.[1] Follow-up PET/CT attenuated corrected (CT-AC) MIP and transaxial PET images [Figure 2a and b] showed low-grade FDG uptake (uptake less than liver) in residual skin thickening (score 3/5)-suggestive of complete response (broad black arrow). Non-AC (NAC) MIP [Figure 2c] and transaxial PET images [Figure 2d] showed moderate FDG uptake (uptake more than liver) in the abdominal wall skin thickening (score 4/5)-suggestive of partial response (arrowhead). Biopsy done for confirmation was consistent with findings of residual disease on NAC images. Moving from “PET only” era to an era of “PET with integrated CT” has led to better disease characterization, decreased scan time, and less patient discomfort. CT, in addition to providing anatomical information, is also used for attenuation correction of PET images. CT-AC restores true tumor count density and permits accurate quantitative assessment for monitoring therapy. However, CT-AC has been shown to reduce lesion's contrast and detection sensitivity, especially in superficial lesions. Phantom studies demonstrated that NAC PET images have better lesion contrast compared to CT-AC images, independent of lesion localization, tissue type, and distance from body surface.[2] Another experimental study showed that statistical noise in the measured attenuation correction factors can increase noise levels in images and reduce effective total counts in emission scan.[3] These observations were validated clinically in patients with lymphoma and carcinoma breast where NAC images were shown to be diagnostically superior for evaluating superficial lesions, and AC images were deemed better for analyzing deep-seated lesions.[45] Interim response evaluation in lymphoma in PET/CT is usually done using quantitative visual 5 point scoring where the uptake intensity with respect to liver rather than standard uptake value threshold is utilized.[1] Hence, in a superficial disease like primary cutaneous lymphoma, where the lesion uptake differs on CT-AC and NAC, analyzing NAC images separately is of important, as shown in our case.

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Figure 1

Baseline staging positron emission tomography/computed tomography showed fludeoxyglucose uptake in the right anterior abdominal wall on maximum intensity projection image (a - thin narrow black arrows) with rest of the scan unremarkable. Fused transaxial positron emission tomography/computed tomography images (b) and positron emission tomography images (c) fludeoxyglucose uptake in the skin thickening in the right anterior abdominal wall

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Figure 2

Follow-up positron emission tomography/computed tomography attenuated corrected maximum intensity projection (a) and transaxial positron emission tomography images (b) showed low-grade flurodeoxyglucose uptake (uptake less than liver) in residual skin thickening (score 3/5)-suggestive of complete response (broad black arrow). Nonattenuated corrected maximum intensity projection (c) and transaxial positron emission tomography images (d) showed moderate fludeoxyglucose uptake (uptake more than liver) in the abdominal wall skin thickening (score 4/5)-suggestive of partial response (arrowhead)

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